Yeasts have been used for production of products that use naturally produced pyruvate as a starting substrate in their biosynthetic pathways. To enhance production of such products, yeasts have been engineered by expressing enzymes to alter endogenous biosynthetic pathways or introduce new pathways, and/or by disrupting expression of endogenous enzymes to alter metabolite flow. Introduced pathways that use cellular pyruvate include pathways for production of isomers of butanol, which are important industrial chemicals, useful as fuel additives, as feedstock chemicals in the plastics industry, and as foodgrade extractants in the food and flavor industry.
Disruption of pyruvate decarboxylase has been used to increase availability of pyruvate for pathways to produce desired products. For example, US20070031950 discloses a yeast strain with a disruption of one or more pyruvate decarboxylase or pyruvate dehydrogenase genes and expression of a D-lactate dehydrogenase gene, which is used for production of D-lactic acid. US2005/0059136 discloses glucose tolerant C2 carbon source-independent (GCSI) yeast strains with no pyruvate decarboxylase activity, which may have an exogenous lactate dehydrogenase gene. Nevoigt and Stahl (Yeast 12:1331-1337 (1996)) describe the impact of reduced pyruvate decarboxylase and increased NAD-dependent glycerol-3-phosphate dehydrogenase in Saccharomyces cerevisiae on glycerol yield. US Patent Application Publication No. 20090305363 discloses increased conversion of pyruvate to acetolactate by engineering yeast for expression of a cytosol-localized acetolactate synthase and substantial elimination of pyruvate decarboxylase activity.
Reducing glucose repression has been used to improve respiratory capacity of yeast for increased biomass production. W0199826079 discloses overexpression of the Hap1 transcription factor to reduce glucose repression, resulting in increased respiratory capacity and increased biomass production. EP1728854 discloses a process for biomass production using yeast overexpressing the Hap1 transcription factor grown in aerobic conditions. Functional deletion of the HXK2 (hexokinase2) gene has been used to reduce glucose repression. Disclosed in WO2000061722 is production of yeast biomass by aerobically growing yeast having one or more functionally deleted hexokinase2 genes or analogs. Rossell et al. (Yeast Research 8:155-164 (2008)) found that Saccharomyces cerevisiae with a deletion of the HXK2 gene showed 75% reduction in fermentative capacity, defined as the specific rate of carbon dioxide production under sugar-excess and anaerobic conditions. After starvation, the fermentation capacity was similar to that of a strain without the HXK2 gene deletion. Diderich et al. (Applied and Environmental Microbiology 67:1587-1593 (2001)) found that S. cerevisiae with a deletion of the HXK2 gene had lower pyruvate decarboxylase activity.
There remains a need to improve growth and product production during fermentation of yeasts that have increased pyruvate availability due to reduction or elimination of pyruvate decarboxylase activity.